chromium: remove preferLocalBuild

It's another attempt to fix chromium builds.

See http://hydra.nixos.org/build/26086977/nixlog/4/raw

Unpacking sources is actually taking more than 2h so build fails.
Instead, rather build it remotely and then copy over the output as
we don't have limits for download time.

See 089bdce6

 for reference

cc @aszlig

(cherry picked from commit cef54e7d67870ff68c9787ff60cd50ca4bf1d8af)
Signed-off-by: Domen Kožar <domen@dev.si>